Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on a hub of a spindle motor for rotation at a constant high speed about a rotational axis. Information is stored on one or more surfaces of the disc or discs in a plurality of concentric circular tracks. An aggregate of tracks on the surfaces of a disc or discs at a given radial position from the rotational axis is referred to as a cylinder. Data is written to, and read from, the tracks via transducers (“heads”) mounted to a radial actuator, which positions the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor (VCM) to position the heads with respect to the disc surfaces. Normally, the VCM includes a coil mounted on the side of the actuator body opposite an array of permanent magnets which are held above and/or below the coil on upper and/or lower magnet plates, respectively. When a controlled current is passed through the coil, a magnetic field is generated. The generated electromagnetic field interacts with the magnetic field of the permanent magnets thus causing the coil to move relative to the magnets. As the coil moves, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
A closed-loop servo system is typically used to control the position of the heads with respect to the disc surfaces. More particularly, during a track seek operation, a servo controller in the servo system receives the address of the destination track and generates a position signal, which is amplified by a transconductance amplifier and presented to the coil of the VCM as a driving current. The driving current causes the actuator, and thus the head, to initially accelerate and then subsequently decelerate as the head nears the destination track. At some point towards the end of the deceleration of the head, the servo system will transition to a settle mode during which the head is settled onto the destination track and, thereafter, the servo system causes the head to follow the destination track in a track following mode.
During the track following mode, servo information is read that provides a position error signal indicative of the position of the head relative to a center line of the track. The position error signal is used, when necessary, by the servo controller to generate a correction signal that in turn is provided to the amplifier. The transconductance amplifier then amplifies the signal and presents the amplified signal to the coils as a driving current. The driving current then causes then adjust the position of the actuator to correct the position of the head relative to the track.
Generally, the objective of a typical seek operation has been to move the head from the initial track to the destination track in a minimum amount of time (access time). In order to move the head to the destination track in the minimum amount of time, the transconductance amplifier is typically driven into saturation. This assures that a near maximum driving current is passed through the coils and, thus, that a near maximum acceleration occurs to move the heads to the desired track. However, due to fluctuations in the voltage supplied to the transconductance amplifier, coil resistance variations, and back-EMFs generated as the coil passes through the magnetic field, the maximum voltage presented to the transconductance amplifier varies significantly over time. As such, maximum drive current, and thus maximum acceleration, is often not achieved.
As described, a disk drive mechanical structure is composed of multiple mechanical components. Each of these components has various resonant modes that if excited by an external energy source will cause the component to physically move at its natural frequencies of oscillation. If the component is highly undamped (i.e. the resonance is high amplitude, narrow frequency band) it will tend to oscillate with a minimal external driving energy. This oscillation adds to the oscillation in the disc drive, eventually resulting in physical actuator motion and, thus, the data head. Motion in the head causes seek settle problems, off track errors, and potential fly height problems. Additionally, the component oscillations also typically create acoustical noise. These oscillations are often referred to as “resonances.”
As mentioned above, typical disc drives produce a driving current through a voice coil motor (VCM) to drive the data head to the desired position. When a frequency spectrum of driving current is analyzed it is found that the spectrum is composed of frequency components from direct current (DC) all the way up to multiple kilohertz (kHz). If driving current is driving the actuator at the same frequency as the resonant mode of a mechanical component, the energy may be sufficient to excite the mechanical component into oscillation. This is very undesirable and will at least degrade disc drive performance or at worst will cause the servo system to go unstable.
To obtain optimal disc drive performance and acoustic characteristics, it is necessary to reduce or minimize resonances in the mechanical components of the disc drive. One way of doing this is to limit the excitation energy at the natural frequency of oscillations of the mechanical components.
One method typically employed to minimize the mechanical oscillation of this type involves the use of hardware electronic filtering and/or digital filtering of the VCM current via a microprocessor or digital signal processor. Both types of filtering achieve the same overall result; they reduce the driving force energy (i.e. the driving current flowing) at frequencies deemed a concern.
One type of filter that is widely used to remove driving energy at the various resonant modes of the components of a disc drive is known as a notch filter. A notch filter is a band-rejection filter that produces a sharp notch in the frequency response curve of driving current from the amplifier. When a notch filter is activated by the servo control loop, the open loop response ends up a summation of the original response plus the notch filter response. If the notch filter is centered about the frequency where the peak amplitude of the mechanical resonance occurs, then the driving current from the transconductance amplifier at this frequency can be reduced so that there will be little or no energy made available to excite the mechanical structure.
However, while notch filters are typically effective at reducing unwanted frequencies in portions of the track seek operation where the characteristics of the transconductance amplifier are linear and predictable, notch filters are ineffective in those portions of the seek operation where the characteristics of the transconductance amplifier are non-linear, such as when the transconductance amplifier goes into saturation. As such, as currently employed in disc drives, notch filters are ineffective at reducing driving force energy caused by frequencies occurring when the transconductance amplifier is in saturation.